1. Field of the Invention
This invention relates to phase sensitive null detectors and, more particularly, to such detectors used to measure the ratio of the resistivity of water entering a reverse osmosis water purifier to the resistivity of product water leaving a reverse osmosis water purifier.
2. Description of the Prior Art
Reverse osmosis water purification systems are in common use for removing ionic material dissolved in water. These systems generally comprise an enclosed container having input and output ports separated by a semi-permeable membrane, such as cellophane.
As is well known in the art, semi-permeable membranes have the characteristic of equalizing the concentration of metallic ions dissolved in a solvent on either side of the membrane. Thus, if the concentration of ions in a solvent, such as water, is greater on one side of the membrane than on the other, water will pass through the membrane from the side of lesser ion concentration to the side of greater ion concentration. The force causing water to move through the semi-permeable membrane in this manner is frequently referred to as osmotic pressure.
The flow of water that occurs through osmotic pressure can be reversed by increasing the pressure on the fluid on the side of greater concentration. In this manner, water can be forced through the membrane, leaving behind the metallic ions dissolved therein. Under these circumstances, the semi-permeable membrane acts essentially as a filter for separating the metallic ions from the water in which they are dissolved. This is the principle used in reverse osmosis water purification systems.
It is important to be able to measure the performance of such reverse osmosis water purification systems to insure that the water produced is of sufficient quality. The concentration of metallic ions in water is inversely proportional to the resistance of the water. Thus, the concentration of metallic ions in water can be calculated by measuring the resistance of the water. Such systems are well known and are in common use. However, since reverse osmosis water purification systems normally remove a relatively constant percentage of the dissolved ionic material, the ionic concentration considered acceptable at the product port will vary, depending upon the ionic concentration at the inlet port. In order to determine whether a purification system is operating properly, it is necessary to measure the resistances at both the inlet port and the product port, and to further calculate the percentage of dissolved ionic material being removed. This is basically how prior art water quality monitors function.
Prior art water quality monitors, as described above, are unacceptable for several reasons. First, insofar as a calculation must be made after each reading, instantaneous performance readings are not possible. Second, the calculation requirement dictates that the water quality monitor be manually operated, thereby increasing the cost of making such measurements. Because of the aforementioned disadvantages of prior art water quality monitors, there is a great need for such a monitor which will produce a signal indicative of the percentage of ionic material removed directly from resistivity measurements at the inlet and product ports.